1. Field of the Invention
This invention pertains generally to broadband mirrors, and more particularly to high reflectivity gratings.
2. Description of Related Art
Broadband mirrors (Δλ/λ>15%) with very high reflectivity (R>99%) are essential for numerous applications, including telecommunications, surveillance, sensors and imaging, ranging from 0.7 μm to 12 μm wavelength regimes. For example, in optical integrated circuits, electro-optic modulators play an important role in switching and signal encoding. Ideally, electro-optic modulators have low insertion loss and wide bandwidth. Mirrors are key components in many modulators so that a low insertion loss, broad bandwidth mirror would greatly improve the performance of these modulators.
Among the candidates for mirrors are metal mirrors and dielectric mirrors. Metal mirrors have comparatively large reflection bandwidths but lower reflectivities (R), as they are limited by absorption loss. As a result, they are not suitable for fabricating transmission-type optical devices such as etalon filters.
Dielectric mirrors on the other hand have a lower loss than metal mirrors and therefore can achieve a higher reflectivity. However, the deposition methods are often not precise enough to lead to very high reflectivities. Dielectric mirrors are composed of multi-layer dielectric materials with different dielectric indices, like distributed Bragg reflectors. Distributed Bragg Reflectors (DBR) consist of multiple periods of alternating high and low refractive index layers. The tuning range for a tunable filter made with DBR mirrors is determined by the DBR mirror bandwidth and the maximum allowable mechanical movement, whichever is smaller. These mirrors have low absorption loss, but the modulation depth, bandwidth and band location depend on the refractive index contrast of the constituent materials as well as on the control over the layer thickness.
In order to minimize interface disorder and strain in the multilayer structures, typical combinations of materials often have small refractive index differences, thus resulting in rather small bandwidths (Δλ/λ≈3-9%). The narrow bandwidth limits the tuning range of the electro-optic modulators like the etalon type devices.
For tunable etalon type devices, such as micro-electro-mechanical (MEM) vertical cavity surface emitting lasers (VCSEL), filters and detectors, the tuning range is often limited by semiconductor based distributed Bragg reflectors (DBRs) to Δλ/λ≈3-9%. The challenge of designing a mirror with broadband reflection, low loss and compatibility with optoelectronic processing has yet to be overcome.
Accordingly, there is a need for highly reflective and ultra-broad bandwidth mirror that is low loss, compatible with semiconductor device fabrication processes and scalable with different wavelength regimes for different application requirements. The present invention satisfies that need as well as others and generally overcomes the limitations of the art.